Guild Reissue Rhythm Chief 1100 Hum

[KirkP]

The hum that the OP is suffering from has absolutely nothing to do with the number of coils in the pickup - that affects only interference from magnetic fields. This hum is from the large electric fields produced by mains wiring and picked up capacitively by the guitar or by the player and then transferred capacitively to the guitar...

You’ve helped me realize I was mistaken to focus only on the electromagnetic coupling of noise to the pickup coils and neglect the electrostatic coupling of noise to to the wiring. I think both can be sources of noise. I’ve found that some noises can be reduced by rotating the guitar (presumably those that are inductively coupled to the highly directional coil) and others don’t change much with the guitar’s orientation or have very complex directional properties (presumably those that are capacitively coupled). I’ve tended to ignore the contribution of the wood body to noise, but wood has a significant dielectric constant that can dramatically increase with moisture content, so it likely does. The human body has a much higher dielectric constant and conductivity than reasonably dry wood, so it makes sense that touching metal part on the guitar that’s grounded (such as the strings) would tend to reduce capacitively-coupled noise sources, but perhaps not do much to reduce noises that are inductively coupled to the coil.

Here’s a long paper on the dielectric properties of wood vs. humidity that I doubt anyone here needs to read.
https://www.fpl.fs.fed.us/documnts/fplrp/fplrp245.pdf

Someone objected to ambiguity of the word “ground”, so I’ll make it clear I’m referring to the shield of the instrument cable.

I don’t know how useful this is to the OP, but it’s a difficult topic.

[aquin43]

An example: I had an archtop with a built in humbucker with tone and volume. The output jack was on the lower bout as usual. All of the wiring was neatly done with shielded wire, the pots had metal cans which were grounded and the strings also were connected to ground (or common) The output jack was a simple Switchcraft type. Everything was as neat as could be.

When placed in playing position the guitar had a low level hum that got much louder as the volume control was turned down. It turned out that touching the wood of the guitar near the output jack induced enough voltage inside the guitar to couple into the contact spring of the jack, even though it was separated from the wood by the jack barrel and the ground contact on the jack. I made a very thin 3cm radius conducting disc from copper foil stuck to paper and placed it as a washer inside the guitar between the jack and the wood. With that in place and connected to the shield, the guitar became completely hum free, even without touching the strings.

To me that shows how easily hum can get into the system by capacitive coupling.

Another problem I have observed with some not very well constructed valve amps is that there can be hum coupled directly to the grid of the first valve that relies on the guitar to short it out. When the guitar volume is at full, the low impedance of the pickup does the job, but when the guitar volume is turned down the output impedance of the guitar rises and the hum appears.

[aquin43]

And I'm not sure why you keep insisting single pickups aren't noisier than humbuckers. Maybe you're using 'hum' very specifically whereas many use it more generally.

In the context I am speaking of, i.e. electrostatic interference, single coil pickups are not noisier than humbuckers.

Magnetic and electrostatic induction of hum are quite different and independent. A Rhythm Chief 1100 will be susceptible to magnetically induced hum but by itself, i.e. linked directly to an amp, it will not be open to electrostatic hum.

[KirkP]

...It turned out that touching the wood of the guitar near the output jack induced enough voltage inside the guitar to couple into the contact spring of the jack, even though it was separated from the wood by the jack barrel and the ground contact on the jack. I made a very thin 3cm wide conducting disc from copper foil stuck to paper and placed it as a washer inside the guitar between the jack and the wood. With that in place and connected to the shield, the guitar became completely hum free, even without touching the strings. ...

So you effectively “grounded” the wood by increasing the capacitance between the wood and the ground side of the jack. I’ve never thought about doing that. I know techs often put metallic paint or tape on the inside of solid body guitars, calling it a “shield”. It’s certainly not a Faraday cage type shield since it’s not enclosed all the way around, but if you think of it instead as a capacitive coupling to the wood it might make more sense. I’ll keep these possibilities in mind next time I’m trying to solve a noise issue.

[aquin43]

So you effectively “grounded” the wood by increasing the capacitance between the wood and the ground side of the jack. I’ve never thought about doing that. I know techs often put metallic paint or tape on the inside of solid body guitars, calling it a “shield”. It’s certainly not a Faraday cage type shield since it’s not enclosed all the way around, but if you think of it instead as a capacitive coupling to the wood it might make more sense. I’ll keep these possibilities in mind next time I’m trying to solve a noise issue.

That is another way of looking at it. I thought of it more as shielding the space that the jack occupied from the voltage on the wood by diverting the field. The best solution would be a shielded jack.

P.S. I have corrected 3cm 'wide' to 3cm 'radius' in the original post